351 H.0 Carb Problems

[Saint351]

I have a 85 bronco with a 351 h.o and the holley 4 barrel. I just recently rebuilt it and sent it to have the floats tuned by a proffesional that I know and trust. After getting the carb back i put it on checked all my vaccum lines and started the beast up. It idled fine so i took off and the first stop sign I got to i tried getting on the truck and i simply cuts out like before. I have noticed that the truck runs much better then before but I just cant stand the dead spot when I try to get on it. I have heard that that specific carb is kinda a pile but hopefully somewon else might have some ideas.

I also have a few ideas of my own such as electric choke might be bad I know a guy with a 77 that was having that problem and it was his electric choke. I also think that possible that floats might not be set just right and my mechanic told me to bring it back down and he would do it over again for free.

Saint 351

 

[indoz]

Hi Saint, setting your float levels is quite easy, make sure your car is on level ground, let it run for a couple of minutes, shut her down, Take off airfilter, the float bodies are on the front and the rear of the carb, on the passenger side side there is a ***** in each float body about half inch dia, take these out and see if fuel leaks out of the holes, the fuel level should be set so it just touches the bottom of the holes, you can adjust the levels by undoing the locknuts on the set screws which you will find at the top of the float bodies. There could be other problems too, the powervalve could be faulty, jets could be loose or blocked, or basically you might not get enough fuel supply when you hit the engine hard, filters or fuel pump?The recommended jets for this model are Front 69, Rear 71, and the powervalve 6.5.

To check your choke let the engine warm up to operating temp without the airfilter and see if the choke valve opens fully.

Hope this helps you, Henk from Oz

 

[hawk2100n]

Check for water in the fuel filter. It can get in there and starve the engine. i have an 351 ** too and i replaced the carb with a new holley street avenger 670 cfm carb. the engine runs much better. my origional carb warped so it never worked right. take indoz suggestions and replace the fuel filter.

 

[mudbug6483]

What size carb is it, and has it always had that dead spot? It sounds to me like you are loosing vacuum when you romp on it which is a common problem with carbs. Especially if its too big. Does it have mechanical or vacuum secondarys?

 

[Justshootme84]

The original H.O. carb is a 600cfm Holley #4180 (built for MotorCraft) with vacuum secondaries. This carb has often been refered to as the "Holley Bomb" for many reasons, mainly that it gives lots of problems. Since you stated it was professionally rebuilt, I will assume the rebuild kit was a Holley. The cheaper off-brand kits will give more problems due to poor fitment of the gaskets. The stalling or "deadspot" problem is normally due to a worn accelerator pump assembly, improper float levels or sticking floats, and vacum leaks. My original carb was so worn out that even a rebuild didn't solve this problem, so I replaced it with a reman. OEM unit.

I also found several other items that affected the carb that needed fixin'. There is a ported vacuum fitting right in front of the carb on the intake manifold that is plastic and easily broken. This and any other vacuum leaks will bog you down. It's a spaghetti-mess of lines to deal with, so just be sure none are cracked or loose. I found that the electric choke lead kept coming off from the alternator. It has a simple blade clip that fits into a slot in back. I solved this by adding a new connector and bolting it directly to the alternator. This is the power source for the choke, as the wire runs around the radiator to a relay and then to the carb. Be sure the vacuum advance on the distributor is working, too. My idle is set at 600RPM after warm-up.

When properly tuned, the H.O. carb should glue you to the seat upon take-off.

 

[Saint351]

Thank you for all of the advice. I checked my electric choke yesterday in shop class and it seems fine its getting power to the wire that goes on the carb. Im going down to have the floats reset by me mechanic again probably tomorrow so hopefully that does the trick. If that doesnt do it I will start checking vaccum lines I belive there is 3-5 of them if my memory serves me.

Thanks again for all the advice its very much appreciated.

Saint

 

[Seabronc]

If I understand you correctly, you don't have vacuum operated secondaries on this carb so some of this may not apply. What type of linkage is ther to the secondaries?

However, the problem may be caused by the secondaries opening too soon. Note from the Holly rebuild kit: "instructions. The secondaries should not open at low speed. When winging the throttle while in neutral or park, looking at the secondaries they should not open. If they do, the truck will almost surely bog under a load. Nomatter what you do, DO NOT leave the spring out."

On a vacuum operated secondary this is adjusted by changing the Diaphram Spring. If the spring is too weak, the sudden increase in vacuum wil cause the secondaries to open too soon.

Good luck, :-"